High-Si-Content Quenched and Tempered Steel for High-Strength Application

Ni improves toughness in martensitic steels. However, the effect of Si on grain refining and toughness of martensitic steel for high-strength applications including armor grades has not been much studied, which can be also a cheaper alternative replacing Ni and without any micro-alloying addition. With this motivation, an initial laboratory study of a quenched and tempered high-Si steel (0.31C-1.25Si-1.19Mn-0.1Mo-0.29Ni) and low-Si steel (0.32C-0.49Si-0.61Mn-0.46Cr-0.1Mo-0.29Ni) showed that their full thickness Charpy impact energy at - 40 degrees C was 32 +/- 3 and 15 +/- 2 J, respectively, while their microstructures had maximum lath length 15 and 30 mu m, respectively. Based on this, an industrial steel (0.30C-1.27Mn-1.23 Si-0.22 Mo-0.43Ni) was made through electric arc furnace-vacuum arc degassing-continuous casting route with low H-2, O-2 and N-2 contents and hot rolled to 5.2-mm-thick coil, which finally processed to quenched and tempered plate. The air-cooled hot-rolled intermediate plate of 25 mm thickness revealed bainitic microstructure, while the slowly cooled hot-rolled coil sample showed banded microstructure consisting of proeutoctoid ferrite and pearlite. On the other hand, the ferrite bainitic microstructure was revealed when the steel was cooled at 1 degrees C/s through dilatometry using Gleeble 3500C. The bainitic microstructure at low cooling rate indicated high hardenability of this grade. The Ac3 transformation temperature determined by dilatometry was 850 degrees C. Based on this, oil quenching was carried out after heating at 895 degrees C for 7, 12 and 17 min, while low-temperature tempering was done at 250 degrees C for 60 min. The micrographs in these three heating conditions revealed fully martensitic microstructure consisting of fine laths. The hardness of plates varied in the range of 495-506 HBW after tempering at 250 degrees C for 1 h, while yield strength, tensile strength, percentage elongation and percentage of reduction in area varied in the range of 1500-1540, 1780-1820 MPa, 7.37-8.61 and 24.64-27.40, respectively. The Charpy impact energy of sub-size 1/2 thickness specimen varied closely in the range 11.5-13.8 J at - 40 degrees C and their corresponding fracture surfaces showed dimpled ductile rupture morphology that typically belonged to the upper shelf region of Charpy transition curve. The measured hardness, strength, elongation and Charpy energy are in accordance with the requirement of final product. Bright-field TEM image of the tempered steel showed the presence of fine laths and uniformly distributed needle-shaped fine carbides resembling epsilon-Fe2.4C.

Automated summary

The study investigated the impact of Si on martensitic steels, showing its potential as a cheaper alternative to Ni for improving toughness in steel. Laboratory experiments and industrial production resulted in achieving the desired microstructures and mechanical properties of the steel.